1. Field of the Invention
The present invention concerns lathes for carrying out various machining operations on a workpiece, including turning.
2. Description of the Prior Art
A conventional lathe is a machine tool comprising a frame carrying at least one rotatable workpiece support spindle and at least one toolholder carriage movable in translation on the frame along a radial crossfeed axis perpendicular to the axis of the spindle and along a feed axis parallel to the axis of the spindle.
Drive means displace the toolholder carriage along the feed axis and in some cases along the crossfeed axis.
A first mechanical transmission to drive the workpiece support spindle is coupled to a main motor and selectively rotates the workpiece support spindle at an appropriate speed. This first mechanical transmission usually includes a feed gearbox coupled to the main motor and transmitting motion to a feed shaft selectively coupled to the workpiece support spindle, possibly via a spindle clutch.
At least one rotary toolholder spindle can usually be fitted to the toolholder carriage. It is rotated by a tool drive mechanical transmission coupled to the main motor. It carries a milling tool and rotates it at an appropriate tool speed.
Conventional lathes without numerical control include a camshaft driven by the main motor and controlling relative movement of the toolholder carriage and the workpiece support spindle in accordance with a machining cycle that can be adapted as required. A conventional lathe of this kind may have a single spindle or (and preferably) a plurality of parallel spindles mounted on a rotary turret so that the same workpiece can be presented to a plurality of toolholder carriages in succession.
Conventional lathes without numerical control are used to machine a workpiece in various ways, including turning, drilling and milling. At present, however, conventional lathes without numerical control are not able to cut longitudinal threads or helical threads with a large pitch on the workpiece. Longitudinal threads or helical threads with a large pitch have to be made by a process known as generation, for example.
Gears are usually cut on dedicated machines, for example the machine described in document U.S. Pat. No. 1,461,219. Document U.S. Pat. No. 4,034,646 describes a dedicated milling machine. Document U.S. Pat. No. 5,150,518 describes a dedicated machine for cutting internal and external helical threads. However, these dedicated machines cannot carry out a variety of machining operations such as milling, turning and drilling. As a result, a workpiece requiring a threadcutting operation and other machining operations has to be transferred from one machine tool to another, which inevitably causes defects such as lack of concentricity between the various parts of the workpiece. It also increases the cost of producing the part.
Document WO-A-93/16832 describes a lathe on which a toothed turning tool is mounted. During turning, the turning tool is progressively pivoted by rotating it about an axis which is not parallel to the axis of the spindle and additional rotation is imparted to the turning tool, proportionately to the displacement along the feed axis of the toolholder carriage on which the turning tool is mounted.
A device of this kind can cut shapes which are bodies of revolution or helical threads with a relatively short pitch on a workpiece mounted on the spindle of a lathe. However, it appears that this device is not able to cut straight threads (splines) or helical threads with a large pitch on the workpiece.
The same applies to the devices described in documents U.S. Pat. No. 1,406,985, U.S. Pat. No. 1,853,643 and CH-A-370 306, which cut helical threads with a relatively small pitch.
Straight and helical threads can be cut on numerically controlled lathes. These machines are particularly complex, however, and costly and this significantly increases the unit cost of the parts made in this way, to the degree that it is more economic to use conventional dedicated machines to cut the threads.
The problem to which the present invention is addressed is that of designing a new, simple and low-cost mechanical system for manufacturing parts including straight threads or helical threads with a large pitch on a conventional lathe without numerical control. The aim is to exploit the many machining capabilities of a conventional lathe to produce on the same workpiece not only straight or helical threads but also other shapes obtained by drilling, turning and milling. The precision of the part obtained is significantly increased by this means, and the thread cutting operations can be carried out in time that would otherwise be wasted, which substantially reduces production cost without tying up a costly machine.
Another object of the present invention is to obtain full advantage of the components of a conventional lathe so that the number of additional parts to be fitted to the lathe is minimized.